Did Elm change how it uses strings as a fundamental part of certain language constructs? That would break the advanced optimizations. According to the github issues tracker as well as mailing list comments from last year, these were not likely going to change in the language and therefore advanced optimizations are very fragile and likely to break on code in subtle ways.

Note that Elm has always appeared to work on the surface with advanced optimizations, but the bugs would creep in and show up eventually, and the official advice has always been not to use those optimizations.